Sand is actually quite variable. Most of this variability is caused by the different moisture content. 1.25 Tonne per cubic metre is a fairly good approximation. Cement is quite a bit denser at about 1.7 tonne per cubic metre. You will find that a 25kg bag of cement is a tad smaller than 25kg of sand.
Concrete/Screed/Render/Mortar mixes are, invariably, quoted by volume. Therefore, if you ever wanted a 1:1 mix, you'd need slightly more cement (by weight) than sand.
Another point to bear in mind, is that the cement, itself, doesn't actually add to the volume of the final mix. Due to it's fine particle size it simply works it's way into the voids between the grains of sand/balast. For this reason, the final mix (once compacted) generally comes out closer to 2 Tonne per cubic metre.
Hence, my method of approximating requirements, is to consider the filler requied to fill the volume in question, then to calculate the cement to make the appropriate mix.
Coming back to your original question a bag of cement would be approximately 1.25 /1.7 (about 3/4) the volume of a bag of sand of the same weight.
Alternatively, a bag of sand would be approximately 1 and 1/3rd the volume of the same weight of cement.
Just to confuse matters, cement is still available in 25kg and 50kg sizes while sand is usually sold in 40Kg bags, from builders merchants, whereas the sheds generally sell it in 20kg (or even 10kg) bags.
Before you finally order, make sure you check what size bags you're talking about. Not all bags are born equal.
